1. Field of the Invention
The present invention relates to a method of manufacturing a self-aligned guard ring of a photo diode.
2. Description of the Prior Art
A photo diode is a semiconductor device comprising a photoconductivity cell and a junction diode, and is commonly used in photoelectric products, such as cameras and photo sensor devices in scanners. A light-induced current of the photo diode represents a signal, whereas a current present in an absence of light represents noise. The photo diode processes signal data by using a magnitude of a signal-to-noise ratio. In the semiconductor industry, it is often desirable to increase the light-induced current of the photo diode so as to increase the signal-to-noise ratio, and hence to enhance a contrast of the signal. A sensitivity of the photo diode is enhanced and the quality of the photo diode is improved.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a photo diode 30 according to the prior art. The semiconductor wafer 10 includes a silicon substrate 12, and a P-well 14 set on the silicon substrate 12. The photo diode 30 includes a photo sensor 16 set on the surface of the P-well 14. Additionally, the semiconductor wafer 10 includes an isolation layer 18 set on the surface of the silicon substrate 12. The isolation layer 18 surrounds the photo sensor 16 and serves as an insulating material to prevent short-circuiting between the photo sensor 16 and other units.
According to the prior art method of forming the photo sensor 16 of the photo diode 30, an ion implantation process is performed to form an N-type doped region 20 on the surface of the P-well 14. Arsenic (As), with an energy of about 80 KeV and a dosage of about 1015ion/cm2, is used as a major dopant in the ion implantation process. A depletion region 22 for detecting the leakage current is formed along the PN junction between the doped region 20 and the adjacent P-type well 14. In FIG. 1, the area marked with cross-hatching illustrates the depletion region 22.
In the formation of the photo diode 30 according to the prior art, dopants with an energy of about 80 KeV used in the ion implantation process not only create a deeper PN junction depth, but also damage a crystal structure on a surface of the photo sensor 16. Thus, more dark current (that is, current in the absence of light) is generated at the interface between the P-well 14 and the doped region 20 below the isolation layer 18. As a result, the signal-to-noise ratio and the sensitivity of the photo diode 30 are reduced.